If the concept is accepted, I'll add unit tests and documentation updates to make a pull request.

There are several use cases for this operator. For example, I need to allow child companies to inherit partners and products from parent companies, but not siblings or cousins. So, when product or partner is defined on a company, it will be limited to this company and its children.

To implement this without "parent_of" operator, we have to hard code domains for each company. But, using "parent_of", it would be very simple For example, in order or invoice, partner_id would have a domain "[('company_id', 'parent_of', company_id)]". Record rule would be simple too.